When you go to buy a Secure Digital (SD) Card, there are usually a lot of choices. You might stand and stare at the cards, deciding what you need. Or, you could be the type to just grab one to use.
There is a method to the madness of each card and we can cover these so you can make an informed decision.
Form Factors
Initially, there was the SD card that was released in 2000. In 2004, there came the micro-SD card.
Older devices, such as cameras, required the regular SD card and after the release of the micro-SD, there’s a simple adapter that could convert the micro-SD to a regular size SD Card.
Most times, it may be better to buy micro-SD Cards and keep the adapter if a device requires the normal size card.
Card Class
There are currently four card classes, three are in production and one is not. The SD Association, which creates the standards for SD Cards, released the latest class of cards.
NOTE: For more information on the SD Association, go to www.sdcard.org.
The four classes are:
NOTE: There is a download on the SD Association’s website that is a program to format SD Cards. For Linux, the program supports Debian, Ubuntu, Raspberry Pi OS (64-bit), Fedora and Arch Linux. You can find the download page here. The program can determine which Class an SD card is when it detects it, but it also recognizes USB flash drives as SD Cards as well.
Speed Classification
Now, here is where things get interesting. There are six various speed classifications. The speeds all have different meaning. The six types are:
Numeric Value
Sometimes this is not on the Card itself, but on the packaging. The value is the Read speed of the SD Card.
Keep in mind two things. The first is that most values are MB/s which is megabytes per second. Sometimes, you may see Mb/s which is megabits per second. A byte is eight bits, so MB is 8 times larger than Mb. Second, most SD Cards allow for reading and writing at one time, but not both at once. There are exceptions to this rule, but I’ll get into that too.
Class
A large C usually denotes this with the class number inside the C. Most people will tell you that the Class 10 SD Card is the best. Usually this is true, depending on what you’ll use the SD Card. For standard reading and writing for data, this can be true.
The numeric value with the C denotes the standard write speed when writing data sequentially on the card in MB/s. So a C10 has a write speed of 10 MB/s and a C2 SD Card has a write speed of 2 MB/s.
Remember that this is writing sequentially on the SD Card. The data blacks are empty in a space to be filled and there are no used blocks to skip and waste time. If an SD Card is fragmented, the system writes the data blocks here and there with gaps in between the data, which diminishes the speed.
NOTE: For speed’s sake, it is best to defragment SD cards when you need them for something that will require uninterrupted writing speed. If possible, use a blank SD Card for these purposes. For example, when taking video shots on a camera. If the SD Card is fragmented, then it can cause loss of frames, stopping of the recording, crashing of the software, etc.
Ultra High Speed (UHS) Writing Speed
The UHS has two values, 1 and 3. A capital ‘U’ denotes these with a 1 or 3 inside it. You’ll find the UHS 1 or UHS 3 on SD Cards are that Class 10.
These values represent a guaranteed write speed of 10 MB/s (UHS 1) or 30 MB/s (UHS 3).
This is writing speed that can be a minimum speed, but again, this can depend on sequential writing and the device itself that may not support these speeds.
UHS Class
This is not to be confused with the UHS Writing Speed. A Roman Numeral denotes the class (I, II or III) on the SD Card.
The UHS Class deals more with Bus Speed. You can tell these seen when looking at the back of an SD Card. There is usually one row of eight metal contacts on UHS Class I. These eight contacts make a byte. The signals can be ‘reading’ or ‘writing’ data. You can only read or write at once, so if you start a job that is reading data from the card and then start another that is writing, the processes will slow down since the card cannot handle both at the same time.
If you have a UHS Class II card, it has two rows of eight metal contacts (see Figure 1), and a UHS Class III has three rows. You need a special device to handle these extra metal contacts, or it’s seen as a UHS Class I.
FIGURE 1
NOTE: UHS Class II and III are extremely expensive compared to the UHS Class I, but if you have a device that supports it, the speed is dramatically faster.
So, a UHS Class I card can only operate at Half Duplex, meaning it can only read or write at one time. The speed is 104 MB/s.
For a UHS Class II, it can do Full Duplex at 156 MB/s. This means it can read at 156 MB/s and write at 156 MB/s at the same time. Or, it can use both rows of contacts to perform read or write speeds of 312 MB/s.
If you have UHS Class III, it can perform Full Duplex at 312 MB/s and Half Duplex at 624 MB/s.
Let’s look at something here for speeds. If you have an SD Card that is rated as UHS Write speed of U3 and UHS Class I, you can see different numbers for your speed. What this would mean is that the card has a guaranteed minimum speed of 30 MB/s but can perform at speeds up to 104 MB/s.
Video Speed
Most uses for an SD Card will be for video recording. Some SD Cards have a video write speed which signifies a minimum sustained write speeds.
The listing of the video write speeds are ‘V6’, ‘V10’, ‘V30’, ‘V60’ and ‘V90’. The numeric value is in MB/s. An SD Card that is rated as ‘V60’ can write 60 MB/s. This will, of course, be sequential. If an SD card is fragmented, the device will not sustain the speed.
NOTE: For the best video results, start with an empty SD Card before filming or defragment the SD Card before filming. The process will create more sequential blocks so you can keep a sustained write speed longer.
App Performance
Here is where things get a little more interesting.
Most devices today run applications from an SD Card, such as phones, tablets, gaming devices, etc. The App Performance rating allows for devices to run apps faster. The significance is for handling a larger number of files faster.
The standard for measuring the read and write speeds is in ‘Input Output Access per Second’ (IOPS). This is basically the number of tasks performed in a second, which the tasks are reading and writing. The number of IOPS is not a constant value since each task can vary in time. Let’s say, for instance, that we want to read 50 files that are 1 KB in size. The process would occur quickly, but would be slower if the file sizes were larger. Many things could cause IOPS to vary, but the general values hold for performing minor tasks. Keep in mind that these tasks may not be performing reading or writing sequentially. The blocks to be accessed may be more randomly spread across an SD Card.
There are two standards for App Performance which are written as ‘A1’ and ‘A2’. The device needs to support either ‘A1’ or ‘A2’ to benefit from the performance.
The Nintendo Switch supports ‘A1’ and not ‘A2’. If you use an ‘A2’ card, there are no benefits gained at all.
The ‘A1’ chip can support:
Conclusion
There are many speed types of SD Cards. Devices may not support all speed types, but once you know what a device supports, you can make a more informed decision about which SD Card to buy.
The better speed you can get from an SD Card, the better the performance will be for your device.
There is a method to the madness of each card and we can cover these so you can make an informed decision.
Form Factors
Initially, there was the SD card that was released in 2000. In 2004, there came the micro-SD card.
Older devices, such as cameras, required the regular SD card and after the release of the micro-SD, there’s a simple adapter that could convert the micro-SD to a regular size SD Card.
Most times, it may be better to buy micro-SD Cards and keep the adapter if a device requires the normal size card.
Card Class
There are currently four card classes, three are in production and one is not. The SD Association, which creates the standards for SD Cards, released the latest class of cards.
NOTE: For more information on the SD Association, go to www.sdcard.org.
The four classes are:
- SD – Secure Digital
- SDHC – Secure Digital High Capacity
- SDXC – Secure Digital eXtended Capacity
- SDUC – Secure Digital Ultra Capacity
- SD allows for a size of 2GB or less with a file format of FAT12 or FAT16
- SDHC is 2GB to 32GB in size and has a file format of FAT32
- SDXC can range from 32GB to 2TB and have an exFAT format
- SDUC allows for sizes between 2TB and 128TB and requires its own drivers, protocols and hardware (this is not in a physical stage yet)
NOTE: There is a download on the SD Association’s website that is a program to format SD Cards. For Linux, the program supports Debian, Ubuntu, Raspberry Pi OS (64-bit), Fedora and Arch Linux. You can find the download page here. The program can determine which Class an SD card is when it detects it, but it also recognizes USB flash drives as SD Cards as well.
Speed Classification
Now, here is where things get interesting. There are six various speed classifications. The speeds all have different meaning. The six types are:
- Numeric Value – A speed, such as 140 MB/s
- Class – Class 2, Class 4, Class 6 and Class 10 (represented by a large C with the class number inside it)
- Ultra High Speed (UHS) Write Speed – UHS 1 or UHS 3 (represented by a large U with a 1 or 3 inside it)
- UHS Class – I, II or III (represented by a roman numeral or U1, U2 or U3; also shown by the number of rows of metal contacts on the back of SD Card)
- Video Speed - V6, V10, V30, V60 or V90
- App Performance – A1 or A2
Numeric Value
Sometimes this is not on the Card itself, but on the packaging. The value is the Read speed of the SD Card.
Keep in mind two things. The first is that most values are MB/s which is megabytes per second. Sometimes, you may see Mb/s which is megabits per second. A byte is eight bits, so MB is 8 times larger than Mb. Second, most SD Cards allow for reading and writing at one time, but not both at once. There are exceptions to this rule, but I’ll get into that too.
Class
A large C usually denotes this with the class number inside the C. Most people will tell you that the Class 10 SD Card is the best. Usually this is true, depending on what you’ll use the SD Card. For standard reading and writing for data, this can be true.
The numeric value with the C denotes the standard write speed when writing data sequentially on the card in MB/s. So a C10 has a write speed of 10 MB/s and a C2 SD Card has a write speed of 2 MB/s.
Remember that this is writing sequentially on the SD Card. The data blacks are empty in a space to be filled and there are no used blocks to skip and waste time. If an SD Card is fragmented, the system writes the data blocks here and there with gaps in between the data, which diminishes the speed.
NOTE: For speed’s sake, it is best to defragment SD cards when you need them for something that will require uninterrupted writing speed. If possible, use a blank SD Card for these purposes. For example, when taking video shots on a camera. If the SD Card is fragmented, then it can cause loss of frames, stopping of the recording, crashing of the software, etc.
Ultra High Speed (UHS) Writing Speed
The UHS has two values, 1 and 3. A capital ‘U’ denotes these with a 1 or 3 inside it. You’ll find the UHS 1 or UHS 3 on SD Cards are that Class 10.
These values represent a guaranteed write speed of 10 MB/s (UHS 1) or 30 MB/s (UHS 3).
This is writing speed that can be a minimum speed, but again, this can depend on sequential writing and the device itself that may not support these speeds.
UHS Class
This is not to be confused with the UHS Writing Speed. A Roman Numeral denotes the class (I, II or III) on the SD Card.
The UHS Class deals more with Bus Speed. You can tell these seen when looking at the back of an SD Card. There is usually one row of eight metal contacts on UHS Class I. These eight contacts make a byte. The signals can be ‘reading’ or ‘writing’ data. You can only read or write at once, so if you start a job that is reading data from the card and then start another that is writing, the processes will slow down since the card cannot handle both at the same time.
If you have a UHS Class II card, it has two rows of eight metal contacts (see Figure 1), and a UHS Class III has three rows. You need a special device to handle these extra metal contacts, or it’s seen as a UHS Class I.
FIGURE 1
NOTE: UHS Class II and III are extremely expensive compared to the UHS Class I, but if you have a device that supports it, the speed is dramatically faster.
So, a UHS Class I card can only operate at Half Duplex, meaning it can only read or write at one time. The speed is 104 MB/s.
For a UHS Class II, it can do Full Duplex at 156 MB/s. This means it can read at 156 MB/s and write at 156 MB/s at the same time. Or, it can use both rows of contacts to perform read or write speeds of 312 MB/s.
If you have UHS Class III, it can perform Full Duplex at 312 MB/s and Half Duplex at 624 MB/s.
Let’s look at something here for speeds. If you have an SD Card that is rated as UHS Write speed of U3 and UHS Class I, you can see different numbers for your speed. What this would mean is that the card has a guaranteed minimum speed of 30 MB/s but can perform at speeds up to 104 MB/s.
Video Speed
Most uses for an SD Card will be for video recording. Some SD Cards have a video write speed which signifies a minimum sustained write speeds.
The listing of the video write speeds are ‘V6’, ‘V10’, ‘V30’, ‘V60’ and ‘V90’. The numeric value is in MB/s. An SD Card that is rated as ‘V60’ can write 60 MB/s. This will, of course, be sequential. If an SD card is fragmented, the device will not sustain the speed.
NOTE: For the best video results, start with an empty SD Card before filming or defragment the SD Card before filming. The process will create more sequential blocks so you can keep a sustained write speed longer.
App Performance
Here is where things get a little more interesting.
Most devices today run applications from an SD Card, such as phones, tablets, gaming devices, etc. The App Performance rating allows for devices to run apps faster. The significance is for handling a larger number of files faster.
The standard for measuring the read and write speeds is in ‘Input Output Access per Second’ (IOPS). This is basically the number of tasks performed in a second, which the tasks are reading and writing. The number of IOPS is not a constant value since each task can vary in time. Let’s say, for instance, that we want to read 50 files that are 1 KB in size. The process would occur quickly, but would be slower if the file sizes were larger. Many things could cause IOPS to vary, but the general values hold for performing minor tasks. Keep in mind that these tasks may not be performing reading or writing sequentially. The blocks to be accessed may be more randomly spread across an SD Card.
There are two standards for App Performance which are written as ‘A1’ and ‘A2’. The device needs to support either ‘A1’ or ‘A2’ to benefit from the performance.
The Nintendo Switch supports ‘A1’ and not ‘A2’. If you use an ‘A2’ card, there are no benefits gained at all.
The ‘A1’ chip can support:
- Random Read – 1500 IOPS
- Random Write – 500 IOPS
- Sustained Sequential Write – 10 MB/s
- Random Read – 4000 IOPS
- Random Write – 2000 IOPS
- Sustained Sequential Write – 10 MB/s
- Command Queuing – Allows a queue depth of 32 commands to hold and allow them to be executed sequentially. The system can add commands to the queue even if the card is busy with another command.
- Cache Function – If the card is busy with reading or writing, it can hold other data to be written in RAM until the card is idle to write the data from RAM
- Self Maintenance – card can defragment itself when it is idle
Conclusion
There are many speed types of SD Cards. Devices may not support all speed types, but once you know what a device supports, you can make a more informed decision about which SD Card to buy.
The better speed you can get from an SD Card, the better the performance will be for your device.